California-based Deep Space Industries says it has signed a contract to provide water-spraying thrusters for the BlackSky Earth observation satellites that are due to be built in Seattle.

The contract covers an initial block of 20 Comet water-based satellite propulsion systems. The systems expel superheated water vapor as propellant to adjust the attitude of small spacecraft in orbit.

Twenty satellites are scheduled to go into orbit by 2020 in the first phase of an Earth observation effort managed by BlackSky, a subsidiary of Seattle-based Spaceflight Industries. The first satellite, dubbed Global-1, is due for launch this year.

One of the provisions of the deal set up a new company called LeoStella, which is owned 50-50 by Spaceflight Industries and Thales Alenia Space. LeoStella is tasked with building BlackSky’s satellites in Seattle.

Eventually, BlackSky plans to have 60 satellites in low Earth orbit.

“The launch-safe propulsion features of the Comet system are well-aligned with BlackSky’s performance needs to enable affordable and flexible satellite systems,” Nick Merski, vice president of space operations for Spaceflight Industries, said in a news release. “We are looking forward to working with the DSI team on this and future projects.”

Deep Space Industries CEO William Miller agreed that the low-pressure, non-toxic system should work well for what BlackSky has in mind. “Customers like LeoStella are exactly why we developed the Comet propulsion system,” he said.

The contract’s financial terms were not disclosed.

Deep Space Industries says the Comet electrothermal thruster system is the first in what it expects will be a full line of “green” propulsion systems for small satellites.

The Silicon Valley company was founded in 2013 as an asteroid mining venture, and has since opened offices in Florida and Luxembourg as well. The use of water as a propellant meshes with DSI’s asteroid aspirations, because water is regarded as the likeliest resource to be found on asteroids.

DSI’s main rival in the asteroid mining market is Planetary Resources, which is headquartered in Redmond, Wash.